For a painted alcove unit, the usual answer is 18mm MDF for the base cupboard carcass and doors, with shelves of 25mm MDF, 18mm birch plywood or solid timber around 20 to 27mm thick, cut 250 to 300mm deep. If you want a natural wood finish, use solid oak or oak-veneered board for the top and shelves. The decision that matters most is the shelf material against the width of your alcove: 18mm MDF is fine across a narrow alcove but sags visibly across a wide one under books, so either choose a stiffer material, go thicker, or support the shelf on three sides.
The usual build
Most alcove units have a base cupboard, often 750 to 900mm high, with doors and a top that sits a little proud of the carcass. Above it, open shelves run wall to wall on battens fixed to the back and side walls. The cupboard is usually about as deep as the chimney breast, and the shelves shallower again.
You can build the cupboard as a box slightly narrower than the alcove and fill the gaps with scribed strips, or build it in place against battens, which copes better with very uneven walls.
Which material for which part
| Part | Common choice | Why | Watch for |
|---|---|---|---|
| Cupboard carcass | 18mm MDF | Flat, stable, takes paint well, usually the cheapest | Heavy, weak screw holding in the edges, swells if it gets wet |
| Doors | 18mm MDF, plain or routed | Stays flat, paints cleanly | Needs priming, especially cut edges |
| Painted shelves | 25mm MDF or 18mm birch plywood | MDF is easy to paint; plywood is much stiffer | 18mm MDF sags on wider spans |
| Natural wood shelves and top | Solid oak or other hardwood, 20 to 27mm | Looks and wears well, stiff | Moves across the grain with the seasons |
| Veneered shelves and top | Oak-veneered MDF or plywood | Oak look without the movement | Edges need lipping; veneer is thin, so sand carefully |
| Wall battens | PAR softwood, around 18 x 44mm or 25 x 50mm nominal | Cheap, easy to fix | Check it is straight before you buy |
MDF is the standard for painted work because it is flat and has no grain to show through paint. It is weak in stiffness and screw holding, so pre-drill and don’t rely on edge screws for load. Near a kitchen or a damp outside wall, use a moisture-resistant grade.
Birch plywood is stiffer than MDF of the same thickness and holds screws better. Its layered edge can be left on show or covered with a hardwood lip.
Solid softwood or hardwood makes the stiffest shelves for a given thickness. Oak is the usual hardwood. Most alcove shelves are wider than a single oak board, so expect wider pieces to be edge-jointed from two or more boards. Remember that planed (PAR) timber finishes a few millimetres under its nominal size: a board sold as 25mm sawn usually planes to around 20mm, and 32mm sawn to around 27mm. Ask for finished sizes.
Oak-veneered board gives an oak finish on a stable core. Lip the front edge with solid oak so it reads as a thick board.
Moisture and heated rooms
Alcoves often sit near a radiator, so solid wood dries and shrinks across its width in winter; MDF and plywood move much less.
Forest Research guidance, quoting EN 942, gives target moisture contents of 9 to 13 per cent for interior joinery in rooms heated to 12 to 21°C, and 6 to 10 per cent where rooms run above 21°C. Purdue University’s furniture guidance makes the same point: dry wood to suit the room it will live in. So ask for kiln-dried timber meant for interior joinery, not stock stored outside, and bring it into the room for a week or two before you cut it.
Fix a solid top to the carcass with slotted holes or buttons so it can move across its width, rather than gluing it down. Solid tops that dry quickly in a heated home can split at the ends.
How thick do the shelves need to be?
Shelf sag depends on the material, thickness, depth, load and above all the span.
A useful target, used by the Sagulator shelf calculator, is no more than about 1.7mm of sag per metre of span. Its author also notes that wood sags roughly another 50 per cent over time under a steady load, and the calculator only gives the initial figure.
Measured tests on the same site give a feel for the differences. For 19mm (3/4 inch) shelves about 300mm deep, loaded at roughly 45kg per metre with the ends simply resting on supports:
- About 760mm span: MDF sagged around 2mm, plywood about 1.3mm, pine and oak under 1mm.
- About 915mm span: MDF sagged around 5.6mm, plywood about 2.5mm, pine and oak about 1.5mm.
So for a typical alcove of 900 to 1000mm, 18mm MDF is a poor choice for books. Use 25mm MDF, birch plywood or solid timber, and run the figures for your own span in the Sagulator before you buy. Two things help any material:
- Support the back edge. A batten along the back wall as well as at each side stops the shelf bending along its length at the back.
- Add a front lip. A hardwood strip glued along the front edge stiffens the shelf noticeably and hides the batten ends. The Sagulator lets you add this edging to the calculation.
These are guides for domestic shelving, not engineering. For heavy loads such as records, go thicker or add a central support.
Measuring the alcove
Assume no wall is square or plumb. Measure the width at the back and front at each shelf height, and work from the narrowest figure for anything that has to slide in. Measure the depth to the chimney breast face at several heights, and check the floor with a level.
Shelves and the cupboard top need scribing, which means cutting the ends to follow the wall. Order each shelf 20 to 30mm longer and a little deeper than the opening. A cardboard or hardboard template for each shelf is the most reliable way to transfer the shape.
Fixing to the walls
Fix battens and the cupboard back rail to solid masonry with plugs and screws long enough to reach well into the brick behind the plaster. Plasterboard on dabs or stud walls need fixings chosen for that wall, and heavy shelves on a stud wall should go into the studs.
Check for cables and pipes before you drill. Under the UK wiring regulations (BS 7671), cables buried less than 50mm deep in a wall are normally run within 150mm of the top of the wall, within 150mm of a corner where two walls meet, or vertically or horizontally in line with a socket or switch, and on a thin wall those zones can carry through to the other side. An alcove has corners on both sides, which is exactly where side battens go. Older or altered wiring may not follow the rules, so use a detector and treat its reading as a guide rather than a guarantee. Heating pipes to a nearby radiator are also common.
Chimney breasts may hide a flue, and old brick can be soft. Keep fixings into the breast short and fix mainly to the back and outer walls.
Party walls. In terraced and semi-detached houses the back wall of an alcove is often the party wall. In England and Wales, the government’s guidance says drilling into a party wall to fix plugs and screws for ordinary wall units or shelving may be too minor to need a notice under the Party Wall etc. Act 1996, though the test is whether the work could affect the wall’s structural strength.
A sample cutting list
This assumes an alcove measuring 950mm wide at its narrowest and 380mm deep, with a cupboard 900mm high, a carcass built 20mm narrower than the opening, and four shelves.
| Part | Qty | Size (mm) | Material |
|---|---|---|---|
| Carcass sides | 2 | 873 x 360 | 18mm MDF |
| Carcass bottom | 1 | 894 x 360 | 18mm MDF |
| Fixing rails (top back, top front) | 2 | 894 x 100 | 18mm MDF |
| Internal shelf | 1 | 890 x 320 | 18mm MDF |
| Plinth | 1 | 930 x 100 | 18mm MDF |
| Doors | 2 | Measure the opening once the carcass is built | 18mm MDF |
| Scribe fillers | 2 | 873 x 50, scribed to the wall | 18mm MDF |
| Cupboard top | 1 | 980 x 400, scribed to fit | 27mm solid oak or 25mm MDF |
| Shelves | 4 | 980 x 290, scribed to fit | 25mm MDF, 18mm birch ply or 27mm oak |
| Shelf battens (back) | 4 | 950, cut to fit | PAR softwood |
| Shelf battens (sides) | 8 | 270 | PAR softwood |
| Front lips (optional) | 4 | 980 x 40 | Hardwood, to match shelf thickness |
To build your own list, sketch the unit with real dimensions, list every part at its finished size, then group parts by material and thickness. A full sheet of MDF or plywood is 2440 x 1220mm, so lay the parts out on a sheet plan. Allow 10 to 15 per cent extra on solid timber, and a spare shelf length if you are new to scribing.
A note on MDF dust
Cutting and sanding MDF makes a lot of very fine dust, which HSE lists among hazardous wood dusts. Cut outdoors where you can, use extraction, and wear a well-fitted mask rated for fine dust.
Build it: tutorials worth following
Free, independent alcove tutorials are scarce, so for now there is one worth following:
- Building alcove shelves, Carpentry Tips and Tricks: a step-by-step for shelves on battens fixed to the back and side walls, with a tip on scribing each shelf to fit out-of-square walls.
What to ask the merchant
- What is the finished thickness? For PAR timber and for sheet materials, ask for the actual size so your cutting list works.
- Is the solid timber kiln-dried for interior use? Air-dried stock will shrink once it is in a heated room.
- Which grade of birch plywood is this? Face quality varies, and it matters if the edges and faces will be on show.
- Do you have moisture-resistant MDF? Worth it near kitchens or damp outside walls.
- Can you edge-joint oak boards to width? Wide shelves usually need two or more boards glued together.
For more on how to set out a request, see how to specify timber for a quote.
Sources
- The Sagulator, Woodbin (sag target of 1.7mm per metre, extra sag over time, edging option)
- Shelf sag measurements, 12 inch deep shelves, Woodbin (measured sag for MDF, plywood, pine and oak shelves)
- Making the Grade: appearance grading UK-grown hardwood timber, Forestry Commission (Forest Research) (EN 942 moisture content targets for heated interiors)
- The Shrinking and Swelling of Wood and Its Effect on Furniture, Purdue University (drying to the service environment, end splits in tops in heated homes)
- Why contractors need to apply due diligence when installing cables embedded in thin walls or partitions, NICEIC via Professional Electrician (BS 7671 zones within 150mm of the top of the wall and of corners, in line with accessories, cables under 50mm deep, zones carrying through thin walls)
- The Party Wall etc. Act 1996: explanatory booklet, GOV.UK (drilling for shelving usually too minor to need notice, England and Wales only)
- COSHH and woodworkers: key messages, HSE (MDF dust as a hazardous substance, extraction and respirators as controls)